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Tick, Tock: Humans Have a Slow Molecular Clock

Compared to all other living apes, we humans are slow to reproduce. People take nearly twice as long to become sexually mature as chimpanzees and gorillas, and the average time between generations is about 20 years, compared to about 15 years for chimps. These traits are part of what makes us human. Yet a new study suggests that our long generation time evolved fairly recently.

This conclusion is the result of new work comparing the evolutionary "molecular clock" of humans, chimps, and other apes, conducted by evolutionary geneticist Soojin Yi at the Georgia Institute of Technology in Atlanta and colleagues. The molecular clock is the rate at which mutations, and any resulting evolutionary changes, occur in a given species. Because most mutations occur during the creation of new eggs and sperm, species with longer generation times should have slower molecular clocks. The recent sequencing of the chimp genome provided an unprecedented opportunity to test this hypothesis (ScienceNOW, 31 August 2005).

The researchers lined up some 63 million DNA base pairs from the human and chimp genomes, along with shorter genome segments from baboons, gorillas, and orangutans. The analysis, as reported online this week in the Proceedings of the National Academy of Sciences, showed that the human molecular clock ticked about 11% more slowly than that of gorillas. The chimp's evolutionary rate was also much slower, clocking in at 8% less than the gorilla.

Because the large difference in generation times between humans and chimps does not match the small difference between their molecular clocks, modern human generation times must have evolved recently--perhaps as early as 1 million years ago, the team calculates. This estimate is consistent with evidence from paleoanthropology: For example, studies of tooth development of a 1.5 million year old Homo erectus specimen from Kenya show that it had a shorter, chimplike maturation time, while the teeth of 800,000 year old hominid specimens from Spain reflect a pattern more typical of modern humans.

"This paper presents a very exciting development for understanding the rate of molecular evolution in primates," says Michael Steiper, an anthropological geneticist at Hunter College in New York. But Blair Hedges, an evolutionary biologist at Pennsylvania State University in University Park, says that generation time might only be one factor among many that control the molecular clock.